Piezoelectric actuator and fluid ejection head having the same

ABSTRACT

A first common electrode is formed on a vibration plate and to be fixed at a predetermined potential. A first piezoelectric layer is laminated on the first common electrode and having a first thickness. A drive electrode is laminated on the first piezoelectric layer, to which a drive signal is supplied externally. A second piezoelectric layer is laminated on the drive electrode and having a second thickness thicker than the first thickness. A second common electrode is laminated on the second piezoelectric layer and to be fixed at the predetermined potential.

TECHNICAL FIELD

The present invention relates to a piezoelectric actuator having apiezoelectric element, the element serving as a drive source, formed onthe surface of a diaphragm. The present invention also relates to aliquid ejection head incorporating such a piezoelectric element providedon the surface of the diaphragm opposite a pressure chamber and changesthe volume of the pressure chamber by the piezoelectric element.

BACKGROUND ART

A piezoelectric element becomes deformed upon receipt of suppliedelectric energy and is widely used as, e.g., a liquid ejection head, amicropump, or a drive element for use with a sound-generating member (aspeaker or the like). Here, the liquid ejection head ejects dropletsfrom nozzle orifices by inducing pressure fluctuations in liquid storedin a pressure chamber. For instance, the liquid ejection head includes arecording head used in an image recording apparatus such as a printer, aliquid-crystal ejection head used in manufacturing a liquid-crystaldisplay, and a coloring material ejection head used in manufacturing acolor filter. The micropump is a ultra-compact pump capable of pumping atrace amount of liquid and used at the time of, e.g., delivery of atrace amount of chemical.

One important component used in such a liquid ejection head or amicropump is a piezoelectric actuator having a piezoelectric elementprovided on the surface of a diaphragm. The piezoelectric actuator isattached to a pressure chamber formation substrate having a void whichserves as a pressure chamber, thereby partitioning a part of thepressure chamber with the diaphragm. At the time of ejection of dropletsor delivery of liquid, a drive pulse is supplied to the piezoelectricelement in order to deform the piezoelectric element and the diaphragm(e.g., a deformation portion of the pressure chamber), thereby changingthe volume of the pressure chamber.

In relation to the liquid ejection head or the micropump, strong demandexists for high-frequency actuation of the piezoelectric element. Thisis intended for implementing high-frequency ejection of droplets orimproving liquid delivery capability. In order to implementhigh-frequency actuation of the piezoelectric element, compliance of thedeformation portion must be made smaller than that of a conventionalpiezoelectric element, and the amount of deformation of thepiezoelectric element must be made greater than that employedconventionally. The reason for these measures is that a reduction incompliance of the deformation portion leads to an improvement inresponsiveness. The piezoelectric element can be actuated at a frequencyhigher than a conventional frequency. Further, an increase in the amountof deformation of the piezoelectric element leads to an increase in theamount of volumetric change in the pressure chamber. Hence, the quantityof droplet to be ejected and the quantity of liquid to be delivered canbe increased.

A piezoelectric element of multilayer structure has been proposed as anelement which satisfies mutually contradictory characteristics; that is,the compliance of the deformation portion and the amount of deformationof the piezoelectric element. For instance, there has been put forward apiezoelectric element having a structure in which the piezoelectriclayer is formed into a two-layer structure; that is, an upperpiezoelectric body and a lower piezoelectric body, and in which a driveelectrode (individual electrode) is formed at a boundary between theupper piezoelectric body and the lower piezoelectric body. Further, acommon electrode is formed on an exterior surface of the upperpiezoelectric body and an exterior surface of the lower piezoelectricbody (as described on, e.g., Japanese Patent Publication No. 2-289352A,page 6 and FIG. 5; and Japanese Patent Publication No. 10-34924A, page 5and FIG. 9).

Since the piezoelectric element of multilayer structure has a driveelectrode provided at a boundary between the upper piezoelectric bodyand the lower piezoelectric body, the respective layer piezoelectricbodies are provided with electric fields whose intensities are definedby intervals between the drive electrode and the respective commonelectrodes (i.e., the thicknesses of the respective layer piezoelectricbodies) and potential differences between the drive electrode and therespective common electrode. Therefore, when compared with apiezoelectric element of a single layer structure having a single layerof piezoelectric body sandwiched between the common electrode and thedrive electrode, the piezoelectric element can be deformed greatly withthe same drive voltage as a conventional drive voltage even when thethickness of the entire piezoelectric element is increased slightly andthe compliance of the deformation portion is reduced.

However, acquisition of a characteristic which can respond to a recenthigh level of demand cannot be achieved by mere use of the piezoelectricelement of multilayer structure. For this reason, there is noalternative but to use, as an actual product, a piezoelectric element ofsingle structure in which a single layer of piezoelectric body issandwiched between the common electrode and the drive electrode. Thisfailure can conceivably be attributed to various reasons, includinginsufficient stability of deformation in a piezoelectric element and theamount of deformation of the piezoelectric element, as well asinsufficient manufacturing efficiency and insufficient reliability of aproduct.

DISCLOSURE OF THE INVENTION

It is therefore an object of the invention is to enhance stability ofdeformation of a piezoelectric element of multilayer structure, as wellas to enhance deformation efficiency of the piezoelectric element.Further, another object of the invention is to enhance reliability whileimproving manufacturing efficiency.

In order to achieve the above object, according to the invention, thereis provided a piezoelectric actuator, comprising:

-   -   a vibration plate;    -   a first common electrode, formed on the vibration plate and to        be fixed at a predetermined potential;    -   a first piezoelectric layer, laminated on the first common        electrode and having a first thickness;    -   a drive electrode, laminated on the first piezoelectric layer,        to which a drive signal is supplied externally;    -   a second piezoelectric layer, laminated on the drive electrode        and having a second thickness thicker than the first thickness;        and    -   a second common electrode, laminated on the second piezoelectric        layer and to be fixed at the predetermined potential.

With such a configuration, the linearity of deformation of an upperpiezoelectric layer can be made preferable. As a result, deformation ofthe piezoelectric element, which arises at the time of drivingoperation, can be controlled more precisely, thereby enhancing stabilityof deformation.

Preferably, the drive electrode has a first width in a first direction,and the second piezoelectric layer has a second width in the firstdirection which is wider than the first width, so as to cover both endsin the first direction of the drive electrode.

With such a configuration, the drive electrode remains embedded in thepiezoelectric body. Hence, occurrence of atmospheric discharge can beprevented, thereby preventing occurrence of faulty operation. Theconfiguration prevents occurrence of a failure, such as occurrence of ashort circuit between a drive electrode and another electrode whichwould otherwise be caused during manufacturing operation or when thepiezoelectric actuator is in use.

According to the invention, there is also provided a piezoelectricactuator, comprising:

-   -   a vibration plate;    -   a first common electrode, formed on the vibration plate and to        be fixed at a predetermined potential;    -   a first piezoelectric layer, laminated on the first common        electrode and having a first width in a first direction;    -   a drive electrode, laminated on the first piezoelectric layer,        to which a drive signal is supplied externally;    -   a second piezoelectric layer, laminated on the drive electrode        and having a second width in the first direction which is wider        than the first width; and    -   a second common electrode, laminated on the second piezoelectric        layer and to be fixed at the predetermined potential.

With such a configuration, manufacturing efficiency can be enhanced, andoccurrence of a failure such as a short circuit or atmospheric dischargecan also be prevented.

Preferably, the drive electrode has a third width in the first directionwhich is narrower than the second width such that both ends in the firstdirection of the drive electrode is covered by the second piezoelectriclayer.

According to the invention, a piezoelectric actuator, comprising:

-   -   a vibration plate;    -   a first common electrode, formed on the vibration plate and to        be fixed at a predetermined potential;    -   a first piezoelectric layer, laminated on the first common        electrode;    -   a drive electrode, laminated on the first piezoelectric layer,        to which a drive signal is supplied externally;    -   a second piezoelectric layer, laminated on the drive electrode        and having and having a first width in a first direction; and    -   a second common electrode, laminated on the second piezoelectric        layer and to be fixed at the predetermined potential, the second        common electrode having a second width in the first direction        which is substantially identical with the first width.

With such a configuration, the entirety of an upper piezoelectric layercan be deformed, thereby improving deformation efficiency of thepiezoelectric element.

According to the invention, there is also provided a liquid ejectionhead, comprising any one of the above piezoelectric actuators such thatthe vibration plate constitutes a part of a chamber communicated with anozzle orifice from which a liquid droplet is ejected.

Preferably, the chamber has a first width in a first direction, and thesecond piezoelectric layer has a second width in the first directionwider than the first width.

Preferably, the chamber has a first width in a first direction and thefirst piezoelectric layer has a second width in the first directionwider than the first width.

With such a configuration, the width of the drive electrode can bebroadened to the greatest possible extent, and the amount of deformationof the piezoelectric element can be increased correspondingly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view for describing a basic structure of ahead main body;

FIG. 2 is a plan view of the head main body when viewed from a nozzleplate;

FIG. 3 is a cross-sectional view of an actuator unit according to afirst embodiment of the invention when viewed in a longitudinaldirection of a pressure chamber;

FIG. 4 is a cross-sectional view of the actuator unit according to thefirst embodiment of the invention when viewed in a transverse directionof a pressure chamber;

FIG. 5 is a plan view for describing a recording head having a pluralityof head main bodies;

FIG. 6 is a cross-sectional view of an actuator unit according to asecond embodiment of the invention when viewed in a transverse directionof a pressure chamber; and

FIG. 7 is a cross-sectional view of an actuator unit according to athird embodiment of the invention when viewed in a transverse directionof a pressure chamber.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the invention will be described hereinbelow by referenceto the accompanying drawings. Here, the embodiments will be described bytaking, as an example, a recording head (a kind of liquid ejection head)provided in an image recorder such as a printer or a plotter. Forinstance, as shown in FIG. 5, the recording head has a plurality of headmain bodies 1 which are attached to a mount base 61.

The basic structure of the head main body 1 will first be described.

As shown in FIG. 1, the head main body 1 is formed substantially from aflow passage unit 2 and an actuator unit 3. The flow passage unit 2 isfabricated from a supply port formation substrate 6 having formedtherein through holes which are to act as ink supply ports 4, andthrough holes which are to constitute portions of nozzle communicationports 5, an ink chamber formation substrate 8 having formed thereinthrough holes which are to act as a common ink chamber 7, and throughholes which are to constitute the portions of the nozzle communicationports 5, and a nozzle plate 10 having formed therein nozzle orifices 9oriented in a secondary scanning direction (i.e., a direction orthogonalto a primary scanning direction in which a recording head is to move).The supply port formation substrate 6, the ink chamber formationsubstrate 8, and the nozzle plate 10 are formed by pressing, forexample, a stainless steel plate. The flow passage unit 2 is fabricatedby placing the nozzle plate 10 on one surface of the ink chamberformation substrate 8 (e.g., a lower surface in the drawing) and thesupply port formation substrate 6 on the other surface of the same(e.g., an upper surface in the drawing), and bonding together the supplyport formation substrate 6, the ink chamber formation substrate 8, andthe nozzle plate 10. For instance, the flow passage unit 2 is fabricatedby bonding together the members 6, 8, and 10 by use of, e.g., asheet-shaped adhesive.

As shown in FIG. 2, the nozzle orifices 9 are formed in a plurality ofrows at predetermined pitches. Rows of nozzles 11 are formed from theplurality of nozzle orifices 9 arranged in rows. For example, a row ofnozzles 11 is formed from 92 nozzle orifices 9. Two rows of nozzles 11are formed side by side.

The actuator unit 3 is a member also called a head chip. The actuatorunit 3 comprises a pressure chamber formation substrate 13 having formedtherein through holes (or voids) which are to constitute pressurechambers 12, a diaphragm 14 for partitioning portions of the respectivepressure chambers 12; a cover member 16 having formed therein throughholes which are to constitute portions of supply-side communicationports 15, and through holes which are to constitute portions of thenozzle communication ports 5; and a piezoelectric element 17 serving asa drive source. With regard to the thicknesses of the members 13, 14,and 16, the pressure chamber formation substrate 13 and the cover member16 preferably assume a thickness of 50 μm or more, more preferably, 100μm or more. The diaphragm 14 preferably assumes a thickness of ˜50 μm orless, more preferably, 3 to 12 μm.

In the actuator unit 3, the diaphragm 14 and the piezoelectric element17 constitute a piezoelectric actuator of the invention. The diaphragm14 is a kind of support member on which the piezoelectric element 17 isto be provided.

The actuator unit 3 is made by bonding the cover member 16 to onesurface of the pressure chamber formation substrate 13 and the diaphragm14 to the other surface of the same, and by forming the piezoelectricelement 17 on the surface of the diaphragm 14. Of these members, thepressure chamber formation substrate 13, the diaphragm 14, and the covermember 16 are made from ceramics, such as alumina or zirconia, bysintering.

The pressure chamber formation substrate 13, the diaphragm 14, and thecover member 16 are bonded together in accordance with the followingprocedures. First, ceramic slurry is prepared from ceramic material, abinder, a liquid medium, or the like. Next, a green sheet (i.e., a sheetmaterial which has not yet been sintered) is formed from the slurrythrough use of a common apparatus such as a doctor blade apparatus or areverse roll coater. Subsequently, the green sheet is subjected toprocessing, such as cutting or punching, thereby forming requiredthrough holes. Thus, sheet-shaped precursors for the pressure chamberformation substrate 13, the diaphragm 14, and the cover member 16 areformed. The sheet-shaped precursors are laminated and sintered, therebyintegrating the sheet-shaped precursors into a single sheet-shapedmember. In this case, since the respective sheet-shaped precursors aresintered integrally, special bonding operation is not required.Moreover, a high sealing characteristic can also be achieved at joinedsurfaces of the respective sheet-shaped precursors.

The pressure chambers 12 and the nozzle communication ports 5, which areequal in number to units, are formed in one sheet-shaped member.Specifically, a plurality of actuator units (head chips) 3 are formedfrom one sheet-shaped member. For instance, a plurality of chip areas,which are to become single actuator units 3, are set in a matrix patternwithin one sheet-shaped member. Required members, such as thepiezoelectric element 17, are formed in each chip area. The sheet-shapedmember (i.e., a ceramic sheet) on which the required members are formedis sliced for each chip area, thereby producing a plurality of actuatorunits 3.

The pressure chamber 12 is a hollow section which is elongated in thedirection orthogonal to the row of nozzles 11, and a plurality ofpressure chambers 12 are formed so as to correspond to the nozzleorifices 9. Specifically, as shown in FIG. 2, the pressure chambers 12are arranged in rows aligned with the row of nozzles. One end of eachpressure chamber 12 is in communication with the corresponding nozzleorifice 9 by way of the nozzle communication port 5. The other end ofthe pressure chamber 12, on the side opposite the nozzle communicationport 5, is in communication with the common ink chamber 7 by way of thesupply-side communication port 15 and the ink supply port 4. A part ofthe pressure chamber 12 is partitioned by the diaphragm 14.

Here, the piezoelectric element 17 is a piezoelectric element ofso-called flexural oscillation mode and is provided, for each pressurechamber 12, on the surface of the diaphragm 14 opposite the pressurechamber 12. The width of the piezoelectric element 17 is determined withreference to that of the pressure chamber 12, and the piezoelectricelement 17 is somewhat greater in length than the pressure chamber 12.More specifically, the piezoelectric element 17 is formed so as to coverthe pressure chamber 12 in the longitudinal direction thereof. Forinstance, as shown in FIG. 3, the piezoelectric element 17 has amultilayer structure formed from a piezoelectric body layer 31, a commonelectrode 32, a drive electrode 33, and the like. The piezoelectric bodylayer 31 is sandwiched between the drive electrode 33 and the commonelectrode 32. The detailed structure of the piezoelectric element 17will be described later in detail.

A drive signal supply source (not shown) is electrically connected tothe drive electrode 33. The common electrode 32 is controlled to a givenearth potential. When a drive signal is supplied to the drive electrode33, an electric field whose intensity is related to a potentialdifference between the drive electrode 33 and the common electrode 32develops. Since the electric field is imparted to the piezoelectric bodylayer 31, the piezoelectric body layer 31 becomes deformed in accordancewith the intensity of the imparted electric field. More specifically, asthe electric potential of the drive electrode 33 increases, thepiezoelectric body layer 31 contracts in the direction orthogonal to theelectric field, thereby deforming the diaphragm 14 such that the volumeof the pressure chamber 12 is reduced. In contrast, as the electricpotential of the drive electrode 33 decreases, the piezoelectric bodylayer 31 expands in the direction orthogonal to the electric field,thereby deforming the diaphragm 14 such that the volume of the pressurechamber 12 is increased.

The actuator unit 3 and the flow passage unit 2 are joined. Forinstance, a sheet-shaped adhesive is interposed between the supply portformation substrate 6 and the cover member 16. In this state, pressureis applied to the actuator unit 3 toward the flow passage unit 2,whereupon the actuator unit 3 and the flow passage unit 2 are bondedtogether.

In the head main body 1 having such a construction, a continuous inkflow passage is formed for each nozzle orifice 9 so as to extend fromthe common ink chamber 7 to the nozzle orifice 9 by way of the inksupply port 4, the supply-side communication port 15, the pressurechamber 12, and the nozzle communication port 5. When the actuator unitis in use, the inside of the ink flow passage is filled with ink (a kindof liquid). A corresponding pressure chamber 12 expands or contracts bydeforming the piezoelectric element 17, thereby causing pressurefluctuations in the ink stored in the pressure chamber 12. Bycontrolling the ink pressure, the nozzle orifice 9 can be caused toeject an ink droplet. For instance, if the pressure chamber 12 having astationary volume is subjected to abrupt contraction after having beeninflated, the pressure chamber 12 is filled with ink in association withinflation of the pressure chamber 12. By subsequent abrupt contraction,the ink stored in the pressure chamber 12 is pressurized, whereupon anink droplet is ejected.

Here, high-speed recording operation involves a necessity for ejecting alarger number of ink droplets within a short period of time. In order tosatisfy this requirement, the compliance of the diaphragm 14 and that ofthe piezoelectric element 17 (i.e., a deformed portion of the pressurechamber 12), both elements partitioning the pressure chamber 12 and theamount of deformation of the piezoelectric element 17 must be taken intoconsideration. More specifically, as the compliance of the deformedportion becomes greater, responsiveness of the pressure chamber todeformation is deteriorated, thereby posing difficulty in driving therecording head at a high frequency. In contrast, as the compliance ofthe deformed portion becomes smaller, the deformed portion becomes moredifficult to deform, whereupon the amount of contraction of the pressurechamber 12 becomes smaller and the volume of one ink droplet is alsodecreased.

From this viewpoint, in the case of a recording head employing apiezoelectric element of flexural oscillation mode which has alreadybecome commercially practical, there is employed a piezoelectric elementof monolayer structure formed by interposing a single layer ofpiezoelectric body between a common electrode and a drive electrode. Thepiezoelectric element has a maximum response frequency of about 25 kHzand a maximum ink droplet volume of about 13 pL (picoliters).

In the embodiment, the compliance of the deformed portion is reduced byuse of the piezoelectric element 17 of multilayer structure. Further,the structure of the piezoelectric element 17 is improved, therebyenabling efficient ejection of a required quantity of ink droplet whileenhancing stability of deformation of the piezoelectric element 17. Thefollowing description explains this point.

First, the structure of the piezoelectric element 17 is described indetail. As shown in FIG. 3; the piezoelectric body layer 31 is formedfrom an upper piezoelectric body (i.e., an outer piezoelectric body) 34and a lower piezoelectric body (i.e., an inner piezoelectric body) 35,which are stacked one over another. The common electrode 32 is formedfrom an upper common electrode (i.e., a common outer electrode) 36 and alower common electrode (i.e., a common inner electrode) 37. The commonelectrode 32 and a drive electrode (individual electrodes) 33 constitutean electrode layer.

Here, the terms “upper (outer)” and “lower (inner)” denote positionalrelationships with reference to the diaphragm 14. In other words, theterms denote positional relationships with reference to the surface ofthe piezoelectric element 17 joined to the diaphragm 14 (which can alsobe referred to as an operating surface to be used for deforming thepiezoelectric element 17, to thereby produce an output). The term “upper(outer)” denotes the surface of the piezoelectric element distant fromthe diaphragm 14, and the term “lower (inner)” denotes the surface ofthe same close to the diaphragm 14.

The drive electrode 33 is formed at a boundary between the upperpiezoelectric body 34 and the lower piezoelectric body 35. The lowercommon electrode 37 is formed between the lower piezoelectric body 35and the diaphragm 14. Further, the upper common electrode 36 is formedon the surface of the upper piezoelectric body 34 opposite the lowerpiezoelectric body 35. Specifically, the piezoelectric element 17 has amultilayer structure comprising, in the order from the diaphragm 14, thelower common electrode 37, the lower piezoelectric body 35, the driveelectrode 33, the upper piezoelectric body 34, and the upper commonelectrode 36. The thickness of the piezoelectric body layer 31 is equalto a total thickness of the upper piezoelectric body 34 and the lowerpiezoelectric body 35 that is, about 20 μm. Further, the total thicknessof the piezoelectric element 17, including the is common electrode 32,is about 23 μm.

The total thickness of the conventional piezoelectric element 17 ofmonolayer structure is about 15 μm. Accordingly, as the thickness of thepiezoelectric element 17 is increased, the compliance of the diaphragm14 becomes smaller correspondingly.

The upper common electrode 36 and the lower common electrode 37 arecontrolled to a given potential regardless of the drive signal. In theembodiment, the upper common electrode 36 and the lower common electrode37 are electrically connected together and controlled to the earthpotential. The drive electrode 33 is electrically connected to the drivesignal supply source as mentioned above and, hence, changes a potentialin accordance with a supplied drive signal. Accordingly, supply of thedrive signal induces an electric field between the drive electrode 33and the upper common electrode 36, and between the drive electrode 33and the lower common electrode 37, wherein the electric fields areopposite in direction to each other.

Various conductors; e.g., a single metal substance, a metal alloy, or amixture consisting of electrically insulating ceramics and metal, areselected as materials which constitute the electrodes 33, 36, and 37.The materials are required not to cause any deterioration at a sinteringtemperature. In the embodiment, gold is used for the upper commonelectrode 36, and platinum is used for the lower common electrode 37 andthe drive electrode 33.

The upper piezoelectric body 34 and the lower piezoelectric body 35 areformed from piezoelectric material containing, e.g., lead zirconatetitanate (PZT) as the main ingredient. The direction of polarization ofthe upper piezoelectric body 34 is opposite that of the lowerpiezoelectric body 35. Therefore, when the drive signal is applied tothe upper piezoelectric body 34 and the lower piezoelectric body 35, thesubstances expand and contract in the same direction and can becomedeformed without any problem. Specifically, the upper piezoelectric body34 and the lower piezoelectric body 35 deform the diaphragm 14 such thatthe volume of the pressure chamber 12 is reduced with an increase in thepotential of the drive electrode 33 and such that the volume of thepressure chamber 12 is increased with a decrease in the potential of thedrive electrode 33.

As shown in FIG. 4, in the embodiment, in order to efficiently deformthe piezoelectric element 17 of multilayer structure, the thickness tp1of the upper piezoelectric body 34 is made greater than the thicknesstp2 of the lower piezoelectric body 35. For instance, the thickness tp1of the upper piezoelectric body 34 is set to 12 μm, and the thicknesstp2 of the lower piezoelectric body 35 is set to 8 μm. By such aconfiguration, the required drive voltage becomes higher by an amountcorresponding to an increase in the thickness of the piezoelectric bodylayer 31. However, the linearity of deformation of the upperpiezoelectric body 34; that is, a characteristic of the upperpiezoelectric body capable of tracking a change in the drive signal, canbe made favorable. Consequently, deformation of the piezoelectricelement 17, which would be induced at the time of driving operation, canbe made stable. Namely, the piezoelectric element 17 can be deformedinto a designed shape. As a result, the volume of the pressure chamber12 can be controlled more precisely, and therefore the piezoelectricelement is suitable for an application involving more elaborate controlof an ejection characteristic; for example, an application tohigh-quality printing.

With regard to the drive electrode 33, in the embodiment, the upperpiezoelectric body 34 is provided so as to be wider than the driveelectrode 33. The upper piezoelectric body 34 covers the entire width ofthe drive electrode 33 in a continuous manner. This is intended forpreventing occurrence of a failure, such as atmospheric discharge or thelike. Specifically, as mentioned previously, an interval between thedrive electrode 33 and the upper common electrode 36 and an intervalbetween the drive electrode 33 and the lower common electrode 37 arevery narrow, on the order of a few microns to tens of microns.Application of a voltage of the order of 30 V to 40 V is required todrive the respective layer piezoelectric bodies 34, 35. For this reason,if both end sections of the drive electrode 33 in a transverse directionthereof have appeared below the layer piezoelectric bodies 34, 35,atmospheric discharge may arise in a hot, humid atmosphere, possiblyinducing faulty operation or a short circuit during manufacturingoperation. When the drive electrode 33 is coated with the upperpiezoelectric body 34 as described in the embodiment, the driveelectrode 33 is embedded in the piezoelectric body layer 31, therebypreventing occurrence of atmospheric discharge or faulty operation.Further, there can be prevented occurrence of a short circuit betweenthe drive electrode 33 and another electrode (e.g., the upper commonelectrode 36 or the lower common electrode 37), which would otherwise becaused during manufacturing operation or when the device is in use.

As shown in an enlarged manner in FIG. 4, the piezoelectric body layer31 (the lower piezoelectric body 35) is provided in an overhung mannerin excess of side edges of the lower common electrode 37. Also, thelower common electrode 37 is made narrower than the width wc of thepressure chamber 12 and is provided within the width of the pressurechamber. As a result, elastic regions Vc, Vc where only the diaphragm 14is situated are formed at both ends of the diaphragm 14 in thetransverse direction thereof. Provision of the elastic regions Vcrenders the diaphragm 14 easier to deform, thus enhancing deformationefficiency.

In the embodiment, an electrode material, which is thinner and moreflexible than electrode materials of other electrodes (e.g., the driveelectrode 33 and the lower common electrode 37), is used for the uppercommon electrode 36. The reason for this is that the upper commonelectrode 36 is deformed to a greater extent than are other electrodes.Specifically, the upper common electrode 36 is formed on the surface ofthe upper piezoelectric body 34 and hence becomes deformed to a greaterextent than are the other electrodes. For this reason, a material whichis softer than those of the other electrodes is used for the uppercommon electrode 36, and/or the thickness of a material layer is madesmaller. As a result, a fracture due to repeated deformation can beprevented. Further, an electrode material having superior conductivityis preferably used so as to prevent occurrence of an excessive increasein electrical resistance, which would otherwise be caused when thethickness of the upper common electrode is reduced.

To be more specific about materials of the electrodes, the upper commonelectrode 36 is formed from gold, and the drive electrode 33 and thelower common electrode 37 are formed from platinum in the mannermentioned above. In relation to the thickness of an electrode, the lowercommon electrode 37 and the drive electrode 33 assume a thickness of 2to 3 μm, whereas the upper common electrode 36 assumes about one-tenththat thickness (e.g., 0.3 μm). By such a configuration, the upper commonelectrode 36 can be deformed so as to follow the piezoelectric element17, thereby preventing a reduction in the amount of deformation of thepiezoelectric element 17. Further, even when subjected to repeateddeformation, the piezoelectric element 17 is not prone to failure, suchas rupture. Moreover, an electric current can be caused to flowefficiently through the upper common electrode 36.

A second embodiment shown in FIG. 6 is characterized in that the upperpiezoelectric body 34 is formed so as to become wider than the innerdimension wc of the pressure chamber 12; that the upper common electrode36 is formed so as to become wider than the lower common electrode 37;and that the upper common electrode 36 is formed over the entire widthof the upper piezoelectric body 34 in a continuous manner.

In the embodiment, the width wp1 of the upper piezoelectric body 34 isformed so as to become wider than the inner dimension wc of the pressurechamber 12 and the width wp2 of the lower piezoelectric body 35 andnarrower than an interval wc″ (an interval wc″ between partitions)between the centers of pressure chamber partitions 38 in a widthwisedirection. Further, the center of the upper piezoelectric body 34 in thetransverse direction is aligned with the center of the pressure chamber12 in the transverse direction. In other words, the upper piezoelectricbody 34 is formed inside of a width range defined between the centers ofthe pressure chamber partitions 38 in the thicknesswise directionthereof (i.e., inside of the range denoted by the partition intervalwc″). As a result, a clearance is provided between adjacent upperpiezoelectric bodies 34, and the piezoelectric elements 17 are providedwithout involvement of contact.

The width (formation width) we1 of the upper common electrode 36 iscaused to match the width wp1 of the upper piezoelectric body 34. Inother words, the upper common electrode 36 is continuously formed fromone end to the other end of the upper piezoelectric body 34 in thetransverse direction thereof. In the embodiment, gold, which hassuperior conductivity and is a soft electrode material, is also used forthe upper common electrode 36, and is formed into a very thin layer ofabout 0.3 μm.

In other respects, the recording head is identical in configuration withthat described in connection with the previous embodiment. Hence, thesame reference numerals are assigned to corresponding portions, andrepetitive explanations are omitted.

In the embodiment, the width wp1 of the upper piezoelectric body 34 iswider than the inner width wc of the pressure chamber 12 in thetransverse direction thereof, and the upper common electrode 36 isformed so as to cover the upper piezoelectric body 34 in a widthwisedirection. An electric field developing between the drive electrode 33and the upper common electrode 36 affects the entirety of the upperpiezoelectric body 34 in a transverse direction. As a result, theentirety of the upper piezoelectric body 34 can be deformed in thetransverse direction thereof. Since the upper piezoelectric body 34 isformed so as to become wider than the pressure chamber (i.e., wider thanthe inner dimension wc), the amount of deformation of the center sectionof the upper piezoelectric body 34 in the transverse direction thereofcan be made greater than in the previous embodiment. Accordingly, thecenter of the diaphragm 14 in the transverse direction of the pressurechamber 12 can be greatly deformed, thereby efficiently transforming thedeformation of the piezoelectric element 17 into a change in the volumeof the pressure chamber 12.

The width we1 of the upper common electrode 36 is made greater than thewidth we3 of the lower common electrode 37. Therefore, the range ofdeformation of the upper piezoelectric body 34 can be made wider thanthe range of deformation of the lower piezoelectric body 35. The centersection of the upper piezoelectric body 34 in the transverse directionthereof can be deformed to a greater extent than can the center sectionof the lower piezoelectric body 35. Since the upper piezoelectric body34 is more distant from the diaphragm 14 than is the lower piezoelectricbody 35, deformation of the upper piezoelectric body 34 is amplified,thereby applying the amplified deformation to the diaphragm 14. Even inthis regard, the center of the pressure chamber 12 in the transversedirection can be deformed greatly.

By such a configuration, the width of the drive electrode 33 can bebroadened to the width of the lower piezoelectric body 35. As mentionedabove, as a result of the width of the drive electrode 33 beingbroadened, an electric field developing between the electrodes can bemade more intense than in the previous embodiment. Therefore, thepiezoelectric element 17 can be deformed to as great an extent aspossible, thus increasing a change in the volume of the pressure chamber12 to as great a level as possible.

In the embodiment, the inner dimension wc of the pressure chamber 12 is160 μm, and a pitch at which the pressure chambers 12 are formed (i.e.,an interval corresponding to wc″ shown in FIG. 6) assumes a value of 210μm. Hence, the width wp1 of the upper piezoelectric body 34 can beincreased to a maximum of about 1.3 times the inner dimension wc of thepressure chamber 12.

Since the width wp1 of the upper piezoelectric body 34 is greater thanthe width wp2 of the lower piezoelectric body 35, the upperpiezoelectric body 34 can be readily formed. Specifically, at the timeof manufacture of the piezoelectric element 17, a paste of electrodematerial (e.g., platinum) which is to constitute a lower commonelectrode 37 is applied over the diaphragm 14 in a predetermined patternby way of a mask, and the thus-applied paste is then sintered. Afterformation of the lower common electrode 37, a paste of piezoelectricmaterial (e.g., lead zirconate titanate) which is to constitute thelower piezoelectric body 35 is applied over the lower common electrode37 in a predetermined pattern by way of the mask, and the thus-appliedpaste is then sintered. Application of a paste and sintering of the sameare repetitively in the same manner, whereby the drive electrode 33, theupper piezoelectric body 34, and the upper common electrode 36 areformed sequentially.

During the formation process, a pattern corresponding to the respectiveupper piezoelectric bodies 34 can be formed so as to become wider thanthe pattern of the lower piezoelectric body 35. Therefore, alignment ofthe mask used for forming the upper piezoelectric body 34 becomesrelatively easy, thereby enabling an attempt to make a manufacturingoperation efficient.

Further, the width wp1 of the upper piezoelectric body 34 is greaterthan the width wp2 of the lower piezoelectric body 35. Hence, the driveelectrode 33 can be covered reliably with the upper piezoelectric body34. As a result, there can be reliably prevented occurrence of afailure, such as a short circuit between the drive electrode 33 and thecommon electrode 32, thereby preventing occurrence of a failure such asan atmospheric discharge.

A third embodiment shown in FIG. 7 is characterized in that the lowerpiezoelectric body 34 is formed so as to become wider than the innerdimension wc of the pressure chamber 12.

In the embodiment, the width wp2 of the lower piezoelectric body 35 isformed so as to become wider than the inner dimension wc of the pressurechamber 12. The individual sections are provided in descending orderfrom the widest section as follows. Specifically, the partition intervalwc″ is the widest, and the width wp1 of the upper piezoelectric body 34and the width we1 of the upper common electrode 36 are the secondwidest. The width wp2 of the lower piezoelectric body 35 and the widthwe2 of the drive electrode 33 are the third widest. The inner dimensionwc of the pressure chamber 12 is the fourth widest. The width we3 of thelower common electrode 37 is the smallest.

Even in the embodiment, the centers of the individual sections in thetransverse direction thereof are aligned with the center of the pressurechamber 12 in the transverse direction thereof. The thickness tp1 of theupper piezoelectric body 34 is greater than the thickness tp2 of thelower piezoelectric body 35. In other respects, the recording head isidentical in configuration with that described in connection with theembodiment. Hence, the same reference numerals are assigned tocorresponding portions, and repetitive explanations are omitted.

In the embodiment, the width we2 of the drive electrode 33 is matchedwith the width of the lower piezoelectric body 35 and set to the largestwidth. Therefore, electric fields developing between the electrodes canbe made more intense, thereby enabling the greatest deformation of thepiezoelectric element 17. As a result, ink droplets can be ejectedefficiently. Even in this embodiment, the upper common electrode 36 isformed so as to cover the upper piezoelectric body 34 in the transversedirection thereof Hence, the amount of deformation of the center sectionof the upper piezoelectric body 34 in the transverse direction can bemade greater than in the previous embodiment. Accordingly, even in theembodiment, deformation of the piezoelectric element 17 can beefficiently transformed into a change in the volume of the pressurechamber 12. In addition, the width wp1 of the upper piezoelectric body34 is grater than the width wp2 of the lower piezoelectric body 35.Hence, the drive electrode 33 can be reliably covered with the upperpiezoelectric body 34. As a result, there can be reliably preventedoccurrence of a failure, such as a short circuit which would otherwisedevelop between the drive electrode 33 and the common electrode 32.Further, occurrence of a failure due to an atmospheric discharge canalso be prevented.

Moreover, in the embodiment, the width wp2 of the lower piezoelectricbody 35 is greater than the inner widthwise dimension wc of the pressurechamber 12. Hence, a deformation portion of the diaphragm 17 is coveredwith the piezoelectric element 17. Therefore, the compliance of thatportion becomes smaller than that of the same portion achieved in theprevious embodiment. As a result of a reduction in compliance, theresponsiveness of the piezoelectric element to deformation is improved.Hence, the piezoelectric element 17 can be driven at a higher frequency.Consequently; ejection of ink droplets at a higher frequency can beachieved. In the embodiment, the width wp2 of the lower piezoelectricbody 35 is set so as to becomes smaller than the width wp1 of the upperpiezoelectric body 34. However, the width wp2 can be increased to thesame width as the width wp1 of the upper piezoelectric body 34.

The above explanations have been provided by reference to an example ofa recording head, which is a kind of liquid ejection head. However, theinvention can also be applied to another liquid ejection head, such as aliquid-crystal ejection head or a coloring material ejection head, aswell as to a piezoelectric actuator of the head. The invention can alsobe applied to a piezoelectric actuator for use with a micropump.

1. A piezoelectric actuator, comprising: a vibration plate; a firstcommon electrode, formed on the vibration plate and to be fixed at apredetermined potential; a first piezoelectric layer, laminated on thefirst common electrode and having a first thickness; a drive electrode,laminated on the first piezoelectric layer, to which a drive signal issupplied externally; a second piezoelectric layer, laminated on thedrive electrode and having a second thickness thicker than the firstthickness; and a second common electrode, laminated on the secondpiezoelectric layer and to be fixed at the predetermined potential.
 2. Apiezoelectric actuator, comprising: a vibration plate; a first commonelectrode, formed on the vibration plate and to be fixed at apredetermined potential; a first piezoelectric layer, laminated on thefirst common electrode and having a first width in a first direction; adrive electrode, laminated on the first piezoelectric layer, to which adrive signal is supplied externally; a second piezoelectric layer,laminated on the drive electrode and having a second width in the firstdirection which is wider than the first width; and a second commonelectrode, laminated on the second piezoelectric layer and to be fixedat the predetermined potential.
 3. The piezoelectric actuator as setforth in claim 1, wherein the drive electrode has a first width in afirst direction, and the second piezoelectric layer has a second widthin the first direction which is wider than the first width, so as tocover both ends in the first direction of the drive electrode.
 4. Thepiezoelectric actuator as set forth in claim 2, wherein the driveelectrode has a third width in the first direction which is narrowerthan the second width such that both ends in the first direction of thedrive electrode is covered by the second piezoelectric layer.
 5. Apiezoelectric actuator, comprising: a vibration plate; a first commonelectrode, formed on the vibration plate and to be fixed at apredetermined potential; a first piezoelectric layer, laminated on thefirst common electrode; a drive electrode, laminated on the firstpiezoelectric layer, to which a drive signal is supplied externally; asecond piezoelectric layer, laminated on the drive electrode and havingand having a first width in a first direction; and a second commonelectrode, laminated on the second piezoelectric layer and to be fixedat the predetermined potential, the second common electrode having asecond width in the first direction which is substantially identicalwith the first width.
 6. A liquid ejection head, comprising thepiezoelectric actuator as set forth in claim 1 such that the vibrationplate constitutes a part of a chamber communicated with a nozzle orificefrom which a liquid droplet is ejected.
 7. The liquid ejection head asset forth in claim 6, wherein the chamber has a first width in a firstdirection, and the second piezoelectric layer has a second width in thefirst direction wider than the first width.
 8. The liquid ejection headas set forth in claim 6, wherein the chamber has a first width in afirst direction and the first piezoelectric layer has a second width inthe first direction wider than the first width.
 9. A liquid ejectionhead, comprising the piezoelectric actuator as set forth in claim 2 suchthat the vibration plate constitutes a part of a chamber communicatedwith a nozzle orifice from which a liquid droplet is ejected.
 10. Theliquid ejection head, as set forth in claim 9, wherein the chamber has athird width in the first direction which is narrower than the secondwidth.
 11. The liquid ejection head as set forth in claim 9, wherein thechamber has a third width in the first direction which is narrower thanthe first width.
 12. A liquid ejection head, comprising thepiezoelectric actuator as set forth in claim 5 such that the vibrationplate constitutes a part of a chamber communicated with a nozzle orificefrom which a liquid droplet is ejected.